I'm looking for a plugin that will allow me to play different notes without affecting the speed samples play at. Does anyone have any good recommendations?
Well, you looking for timestreching plugin. Maybe you want to try this https://xenakios.wordpress.com/paulxstretch-plugin/
The first thing you have to realize is: It is mathematically impossible to do perfect time-stretching. Every time-stretching algorithm has strengths and weaknesses. For example PaulStretch (the one herodotas linked) is awesome for stretching samples by big factors (e.g. making it 10x slower) but it doesn't really work that well for small amounts, which I think you are after. Every time-stretching algorithm has some sorts of artifacts, and this is why samplers generally don't do time-stretching. Time-stretching cannot substitute for proper multi-sampling because for example if you play a note on a piano and then play the same note one octave up, it's not just the duration of the note that changes - the timbre itself changes, and that is something that time-stretching cannot do.
Any professional sample-based synthesizer solves this problem by using multi-samples (which you can also do in OpenMPT), not by time-stretching.
How do you do multi-sampling in OpenMPT? Would you just use multiple tracks, or is there a way to add all the samples to one instrument?
Yes, you can have more than one sample per instrument. You can edit the note to sample assignment on the right-hand side of the instrument editor. If you prefer, you can also define your instruments in the SFZ format (https://sfzformat.com/) and create multisamples that way.
Thanks for putting up with my ignorance. I'll definitely use multisampling.
No worries, it's a common assumption that time-stretching is a silver bullet but it's pretty much the opposite. :)